The present invention relates to a circuit which senses and limits the current conducted by a transistor to maintain the transistor in a safe operating area. More particularly, the invention relates to a circuit which senses the current conducted by the collector of a pass transistor in a voltage regulator circuit and limits the drive current provided to the base of the transistor when the sensed current exceeds a current limit value.
A series voltage regulator circuit requires a minimum voltage differential between the supply voltage and the regulated output voltage in order to function. This voltage differential is known as the dropout voltage of the regulator. For a given supply voltage, the dropout voltage of the regulator limits the maximum regulated voltage which can be supplied to the load. Conversely, for a given output voltage, the dropout voltage determines the minimum supply voltage required to maintain regulation. A voltage regulator having a low dropout voltage is therefore capable of providing a regulated output voltage at a lower supply voltage than can a voltage regulator voltage regulator having a higher dropout voltage. A low dropout voltage regulator can also operate with greater efficiency, since the input/output voltage differential of the regulator, when multiplied by the output current, equals the power dissipated by the regulator in transferring power to the load. For these and other reasons, a voltage regulator circuit having a low dropout voltage has many useful applications, and can improve the performance and reduce the cost of other circuits in which the regulator circuit is used. For example, the improvement in dropout voltage allows the use in power supplies of smaller heat sinks and smaller magnetic devices.
A series voltage regulator circuit controls the load voltage by controlling the voltage drop across a power transistor which is connected in series with the load. To prevent the power supply circuitry and regulator circuit from suffering permanent damage under accidental overload conditions, the regulator circuit typically includes circuitry to sense the current conducted by the transistor, and to limit that current to a predetermined safe maximum value when an overload occurs.
One of the failure mechanisms which can damage a transistor operating at a high power level is a phenomenon known as thermal runaway. Thermal runaway results from thermal instabilities in the transistor which cause localized areas in the transistor to overheat and burn-out at highpower levels. The phenomenon is a function of both the collector current and the collector-emitter voltage. Generally, the collector current necessary to trigger thermal runaway decreases at high voltages, although the exact threshold levels of current and voltage defining the safe operating area of the transistor depend on the design of the transistor, and are usually determined by an empirical process which takes the additional factor of temperature into consideration.
Thus, a current limit circuit used in a voltage regulator should ensure that the power transistor is operated within its safe operating area. Ideally, such a current limit circuit should have a high gain to provide a sharply defined current limit and to avoid a degradation in regulation as the current conducted by the transistor approaches the current limit value.
However, in a 3-terminal integrated circuit voltage regulator, providing such a current limit circuit presents several problems. In such a regulator, the current limit circuit (like other circuitry in the regulator) operates solely off the input/output voltage differential. This requirement is a particularly tight constraint in the case of a low dropout voltage regulator. Also, because the input/output voltage differential varies depending on load and line conditions, a current limit circuit should be capable of limiting current independently of the value of the input/output voltage differential although, as mentioned above, it should also be capable of decreasing the current limit value at increasing input/output voltage differentials to safeguard the power transistor.
In view of the foregoing, it would be desirable to be able to provide a current limiting circuit for a 3-terminal voltage regulator circuit having a low dropout voltage, which does not increase the dropout voltage of the regulator circuit.
It would further be desirable to be able to provide an improved high gain current limiting circuit for such a voltage regulator circuit.
It would also be desirable to be able to provide an improved current limiting circuit for such a voltage regulator circuit which is capable of varying the current limit value in response to changes in the input/output voltage differential.